1. Field of the Invention The present invention relates generally to FM tuners including PLLFM demodulation circuits employing a PLL (Phase Locked Loop), and more particularly, to a FM tuner having bandpass filters of an intermediate frequency stage with variable extraction bandwidths.
2. Description of the Background Art A receiver for television and radio broadcasting includes a tuner for selecting a desired transmission to demodulate a signal transmitted with being tuned in to a carrier frequency thereof.
Particularly, a FM tuner included in a receiver for receiving FM broadcasting generally includes a high frequency amplifying circuit portion including a high frequency amplifier for amplifying a received FM signal of a high frequency, and a mixer for converting a frequency band of the FM signal amplified by the high frequency amplifier into an IF (an intermediate frequency band) based on an oscillation frequency of a local oscillator. The FM tuner further includes BPFs (Band-pass Filters) for extracting a predetermined band component from the FM signal frequency-converted in the mixer, an IF amplifying portion for amplifying the FM signal of the band extracted by the BPF to a fixed level, and a FM demodulation circuit for demodulating the FM signal amplified by the IF amplifying portion.
A so-called PLLFM demodulation circuit employing the PLL is widely used as the FM demodulation circuit.
The PLL is employed for a precise frequency follow. FIG. 1 is a schematic block diagram of the PLL. Referring to FIG. 1, the PLL is a closed loop including a phase comparator 100, a LPF (Low Pass Filter) 101 and a VCO (Voltage Controlled Oscillator) 102. The VCO is an oscillator having an oscillation frequency varied in accordance with an input voltage. An operation of the PLL will be described as follows.
In the case when a frequency output f.sub.0 of the VCO 102 coincides with a frequency of an input wave f.sub.1, and a predetermined relationship is established between a phase of the output f.sub.0 of the VCO 102 and a phase of the input wave f.sub.1, the frequency of the output f.sub.0 is stable because change in an output of the LPF 101. However, an output of the phase comparator 100 changes with a change in the input wave f.sub.1 and is filtered by the LPF 101 to be outputted as a control voltage of the VCO 102. Accordingly, the frequency of the output f.sub.0 of the VCO 102 changes and coincides again with the frequency of the input wave f.sub.1 to be stable. That is, an input frequency is locked in the PLL. Therefore, the output of the LPF 101, i.e., the control voltage of the VCO 102 changes in accordance with the change in the frequency of the input f.sub.1 of the PLL. Thus, a FM signal, which is to be demodulated is inputted to the PLL as the input wave f.sub.1, so that the control voltage of the VCO is derived as a FM detected output in the PLLFM demodulator.
The PLL cannot perform such a lock operation as described above for input waves of all the frequencies. That is, the PLL has a so-called lock range,, namely, a specific input frequency range, so that when the PLL is stable, namely, at a phase locked state that the frequency of the input wave is equivalent to that of an output wave, the phase locked state can be maintained even with a moderate change in the frequency of the input wave. The lock range of the PLL is determined by characteristics of the phase comparator, the LPF, the VCO or the like included in the PLL. Therefore, the PLLFM demodulation circuit employing the PLL has the lock range determined by characteristics of the phase comparator, the LPF, the VCO or the like included therein. Thereby if the frequency of an input signal is within this lock range, the VCO in the PLL operates in accordance with an instantaneous frequency of the input signal, so that FM demodulation is carried out. However, if the frequency of the input signal is out of the lock range, the VCO in the PLL does not follow the instantaneous frequency of the input signal, so that the FM demodulation is not carried out. That is, the PLLFM demodulation circuit serves, as a filter for selectively demodulating only the input signals having frequencies are within the lock range. Thus, the lock range of the PLLFM demodulation circuit in the FM tuner is set to an optimum value for a bandwidth of the BPF at the IF stage so that unnecessary frequency components and noise are removed from the input signals to the PLLFM demodulation circuit by this filter function.
In the FM tuner having the PLLFM demodulation circuit as described above, the bandwidth BW of the BPF provided at a preceding stage of the IF amplifying portion (hereinafter referred to as IFBPF) is set to a Carson band W (=2.multidot.f.sub.m +.DELTA.f+f') determined by a maximum modulation frequency f.sub.m, a frequency deviation .DELTA.f and an energy diffusion signal frequency f' of a FM signal to be inputted, in order to extract a band in which the electric power of the FM signal is substantially concentrated. Therefore, the bandwidth of the IFBPFs varies depending on a FM signal to be transmitted to the FM tuner including the IFBPFs. Accordingly, one of the FM tuners which receive various kinds of FM signals having different maximum modulation frequencies, frequency deviations .DELTA.f and energy diffusion frequencies f', is configured such that the bandwidth of the IFBPFs can be switched in accordance with the frequency of a received signal.
Since, a broadcast frequency of a satellite broadcast has been in greater use, for example, a signal is transmitted as a FM signal with a maximum modulation frequency fm, a frequency deviation .DELTA.f, an energy diffusion frequency f' and the like varying depending on the type of a transponder installed in a communication satellite, a DBS (Direct Broadcast Satellite) tuner for receiving the satellite broadcast includes a plurality of BPFs having different bandwidths as the IFBPFs. The BPF to be employed is switched to another BPF in accordance with the received signal. However, due to problems concerning cost, in a conventional FM tuner with the IFBPFs having variable bandwidths, the lock range of the PLL demodulation circuit is set to an optimum value for the bandwidth of one of the BPFs, which is especially associated with performance of the FM tuner.
In the case of such a FM tuner where the bandwidth of the IFBPFs can be switched to 30 MHz and to 15 MHz, for example, the lock range of the PLL demodulation circuit can only be set to an optimum value corresponding to one of the bandwidths of the BPFs, i.e., 15 MHZ or 30 MHz. As a result, the following problems are caused.
FIG. 2 is a schematic diagram for illustrating disadvantages of the conventional FM tuner including the PLLFM demodulation circuit as described above.
Referring to FIG. 2, in case when the lock range of the PLLFM demodulation circuit is optimized for the bandwidth 30 MHz of the IFBPFs to be set to the frequency f.sub.2 to f.sub.3, for example, the lock range of the PLLFM demodulation circuit does not attain an optimum value because the lock range is too large for the bandwidth of the IFBPFs when the bandwidth of the IFBPFs is switched to 15 MHz. In more, the optimum value of the lock range of the PLL demodulation circuit should be f.sub.2 to f.sub.3 in the bandwidth of 30 MHz and f.sub.4 and f.sub.5 in the bandwidth of 15 MHz. In the case when the lock range of the circuit is optimized for the larger bandwidth (30 MHz), and the bandwidth of the IFBPFs is switched to the smaller value at this time, unnecessary signals of f.sub.2 to f.sub.4 and f.sub.5 to f.sub.3 are demodulated by the PLL demodulator. Therefore, if the lock range of the PLLFM demodulation circuit is optimized for the larger bandwidth of the IFBPFs, the lock range is too large for the bandwidth of the input signal (the bandwidth of the IFBPF) when the bandwidth of the IFBPFs is switched to the smaller bandwidth. Accordingly, the PLLFM demodulation circuit no longer serves as a filter, and thus becomes susceptible to noise and interference due to a signal of an adjacent channel.
In order to eliminate these problems, it is necessary to eliminate signals and noises of unrequired frequency components as much as possible in the FM signal to be outputted from the IFBPFs. In order to meet requirement, a filter must be employed as the IFBPF, which has such a characteristic that a gain for an input signal shows an extremely sharp decrease when the frequency of the input signal drops out of the bandwidth to be extracted. However, the use of such filter with a superior characteristics causes an increase in cost.
Again referring to FIG. 2, in the case when the lock range of the PLLFM demodulation circuit is optimized for the bandwidth 15 MHz of the IFBPFs to be set to the frequency f.sub.4 to f.sub.5, for example, on the contrary to the above case, there is no problem as in the above case when the bandwidth of the IFBPFs is selected to 15 MHz. However, when the bandwidth is selected to 30 MHz, the lock range of the PLLFM demodulation circuit does not attain an optimum value because the lock range is too narrow for the bandwidth of the input signal. If the lock range is extremely too for this bandwidth, it sometimes more narrow than the bandwidth of the IFBPFs. In more detail, if the lock range of the PLLFM demodulation circuit is too narrow for the bandwidth 30 MHz of the IFBPFs, a portion of the signal components extracted by the IFBPFs is not demodulated because the portion is out of the lock range of the PLLFM demodulation circuit. Accordingly, in this case, the same phenomenon occurs as in the case when the bandwidth of the IFBPFs more narrow than an optimum value corresponding to a received signal, namely, the above described Carson band, and thus even necessary signal components are removed in the IFBPFs. That is, due to an inadequate bandwidth of the IFBPFs, so-called truncation noise occurs in a demodulated signal. This noise is caused by distortion occurring in the modulated signal because a portion of the signal components required for demodulation with no distortion occurring in the modulation circuit is not demodulated in the demodulation circuit. If the FM signal to be demodulated is a video signal, the truncation noise appears, as a noise, on a picture to be reproduced by the demodulated signal, which degrades the quality of the reproduced picture. Thus, in the case when the lock range of the PLLFM demodulation circuit is optimized for the more narrow bandwidth of the IFBPF, the lock range is too narrow for the bandwidth of the input signal (the bandwidth of the IFBPFs) when the bandwidth of the IFBPFs is selected to the larger bandwidth and the occurrence of the truncation noise results.
As has been described, in the conventional FM tuner having the IFBPFs with a variable bandwidth, it is impossible to optimize the lock range of the PLLFM demodulation circuit for all the bandwidths of the IFBPFs without introducing a high cost.